It has been already known to use a salt or salts in distillation of a binary system. To add salts is effective especially in distillation of a binary system having an azeotrope. By the addition of the salts, vapor-liquid equilibrium is improved, an azeotropic point disappears, the number of trays to be provided in a distillation column for separating the two components is reduced, and a pure product is separated from the azeotropic mixture. According to known methods, a salt or salts are added in such a manner that (1) a solid salt or salts are added to a reflux, or (2) a solution of a salt or salts is mixed with a reflux. However, in the former method, evaporation and drying steps are necessitated to recover the salt or salts with a high purity from a mixture obtained from the base of a column, and the operation of the entire process becomes complicated. The latter method is also unsuitable when it is desired to take off a product of high purity from the top of the column because the salt or salts solution to be added to the top of the tower contains dissolved or suspended impurities to be removed.
In general, the employment of the salt or salts in the process for recovering hydrous or anhydrous ethyl alcohol from fermentation mash involves the following problems:
(1) It is practically impossible to recover and regenerate the salt or salts with a high purity, because the fermentation mash includes various suspended solids or dissolved impurities therein.
(2) A considerably large amount of steam is needed to remove water for recovering the salt or salts, especially when the concentration of salt or salts is low. In fact, the employment of the salt or salts can lower the reflux ratio and curtail the steam consumption necessary for distillation of the product because the salt or salts can improve the vapor-liquid equilibrium and increase the relative volatility, but the total steam consumption of the entire process is increased.
(3) The treatment of the components of the mash feed other than ethyl alcohol, e.g., fusel oil, initial fractions, etc. becomes difficult by the addition of the salt or salts.
The Hiag method described in U.S. Pat. No. 1,891,593 is known as an industrial method for producing anhydrous ethyl alcohol from hydrous alcohol. According to this method, ethyl alcohol is brought into contact with organic acid salts to effect dehydration. More specifically, hydrous ethyl alcohol of 94 to 95 vol% is obtained from a mash column; the liquid ethyl alcohol is fed to the lower portion of a Hiag dehydrating column; vapor is generated by a reboiler provided to the dehydrating column and introduced into a packed tower so as to be brought into contact with descending mixture of alcohol and the salts in a counter-current manner; and anhydrous ethyl alcohol of 99.5 to 99.9vol% is obtained from the top of the tower, utilizing the absorptivity or adsorptivity of the salts with respect to water. A water-alcohol-salts mixture at the base of the tower is subjected to an evaporation operation to evaporate the alcohol component, and the resultant vapor is directly fed to the base of a rectifying column. On the other hand, the residual mixture of water and salts from which the alcohol component has been removed, is further subjected to an evaporation operation to evaporate water for recovering the salts in the molten form. The so obtained salts are then added to the alcohol at the upper portion of the dehydrating column. This method, however, has such a problem that a very large amount of energy is needed to obtain hydrous alcohol of 94 to 95 vol% from the mash column. This is due to a vapor-liquid equilibrium characteristic of the water-ethyl alcohol system such that the relative volatility is lowered around the azeotropic point and the minimum reflux ratio required for the distillation operation is high.